Bungee Ballista Part 9 PDF Print E-mail
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Thursday, 29 October 2009 00:37

Making a Bungee Ballista Part 9

 

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Shuttle Fail

 (left)  Making the shuttle all out of wood was a bad idea.   The forces on it during the abrupt deceleration at the end of the track are just too much.     A new shuttle had to be created.


 The new shuttle had the piece that rides in the track constructed out of wood.  It was created from a knot-free section of a pine 2x3.    1/4" carriage bolts were used to attach two pieces of aluminum 1x1 square tube down.   Steel u-bolts went through both aluminum tubes for secure attachment to the pulling ropes.

new shuttle start

 



new shuttle side
(left)  Here is the side view of the new shuttle.

 

 (below) A board was bolted to the top of the wheel frame, extending over the last foot of track.  Bungee was pulled tightly so that the shuttle would need to stretch three bungee cords before hitting the rubber stops.   This meant the shuttle could slow down over a space of 6 inches instead of .5 inches, meaning the peak force was probably lower by more than a factor of 10.

  This view also shows the rear of the new shuttle.  Another aluminum tube piece extended out past the track aluminum, so that pieces could ride underneath the angle and hold the rear of the shuttle on the track.

new braking system

 



  Here is the final configuration which worked reliably for over 100 shots.   The shuttle even survived one of the ropes breaking.  The severely unbalanced pulling did no damage to shuttle, and it did not come off the track.

new shuttle on the final design from side



winchy

The winch is a boat trailer which rated at 1500 pounds.  It had to be centered so that the strap would pull the shuttle directly up the center of the track.   The problem was that the handle would then just hit the main beam.   The solution was to raise the winch off the main beam so the winch handle would clear the side.   This was a pain, since those two pieces of wood needed to be held very firmly to the main beam.   There can be hundreds of pounds trying pull the winch off.  Do not use deck screws, they are NOT designed for shear loads.  They are designed to keep boards from pulling vertically up.

 


 


 To help make sure the rope and bungee goes nicely on the wheel rim, a 1x3 chunk of wood was tacked in with deck screws.   Then a piece of pvc pipe was cut, and a counter-sunk hole drilled in the center to attach it to the wood.   The pvc piece was cut to be just the right height to make the rope line up with the center of the rim.   This is especially important to make sure the knot doesn't cause problems as it winds on the wheel.   Some tiny screws were put on the edge of the pvc to keep it from rotating.

Rope Guide

 guide side

 

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Last Updated on Thursday, 29 October 2009 02:36
 

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